Place sliced red onion on the bottom of the baking dish.
Use a paring knife to cut a few slits into the top and bottom of the chuck roast, then place the halved garlic cloves inside. Season the beef extremely generously on all sides with Kosher salt and black pepper, salt and pepper. Place on top of the red onions.
Pour ¼ cup of water around the roast, then cover it tightly with aluminum foil. Transfer to the oven and cook for 3½-4 hours, or until the meat is very tender and shreds easily.
Carefully transfer the roast to a large cutting board, then use tongs or two forks to shred the meat. Return it to the baking dish, then toss it with the onion and juices. Increase oven temperature. Cook uncovered until there are lots of crispy bits on top. Enjoy immediately!